Natural gases discharged to the surface of the ground from high-temperature high-pressure underground areas such as oil and gas fields include: dry gases with one to two carbon atoms such as methane (CH4) and ethane (C2H6), which are in gaseous form even at normal temperature and pressure; natural-gas liquids (NGL) containing components with three to four carbon atoms such as propane (C3H3) and butane (C4H10), which are in gaseous form at normal temperature and pressure but are easily liquefied when pressurized or cooled, and components with five or more carbon atoms such as pentane (C5H12), which is in liquid form at normal temperature and pressure; and non-hydrocarbon gases such as carbon dioxide (CO2).
As described in, for example, Patent Literature 2, such a natural gas is pressurized (to 0.5 to 2 MPa) by a compressor and cooled (to 0 to 20° C.) by a cooling unit to undergo gas-liquid separation in which a part of its natural-gas liquid that is a high-boiling-point hydrocarbon component is liquefied. The natural gas is then raised in temperature by being heated (to 50 to 80° C.) by a heating unit. Thereafter, the carbon dioxide is passed through a carbon-dioxide separation membrane to a lower pressure side, so that the carbon dioxide is separated and removed from the natural gas. As a result, the natural gas is purified.
The composition of natural gas (the proportions of dry gas, natural-gas liquid, carbon dioxide, and the like) differs from one natural-gas source to another. Thus, a natural-gas purification apparatus as mentioned above has its equipment configured according to the composition of the natural gas to balance the apparatus' performance and natural-gas purification cost (including costs such as the raw-material cost, the equipment cost, and the running cost).